Generally, elastomers have an inherent ability to resist compression with a very high amount of resistant force, often exhibiting a much higher spring rate in the compressed state than that of well known spring steel. That is why elastomers have been widely used in applications requiring shock absorption. However, one of the major disadvantages of using only an elastomer as a shock absorber is its inherent characteristic of returning the applied force back into the system thus causing recoil.
Typically, an elastomer is used in conjunction with a steel casing to provide shock absorbing capabilities to reduce the recoil action. U.S. Pat. No. 5,495,923 to M. Bruski et al and U.S. Pat. No. 4,591,030 to R. Antkowiak teach an elastomeric shock absorber having a compressible elastomer member disposed within a cylindrical chamber and abutting the piston surface as described in U.S. Pat. No. 4,591,030 or enclosed about a piston rod as described in U.S. Pat. No. 5,495,923. In either example, the elastomer member is compressed by the piston during shock absorption. The shock absorber taught in U.S. Pat. No. 5,495,923 further contains a variety of flow communication ports to dispose the elastomer material from the first chamber to the second chamber during shock absorption and enable return flow during the return motion of the piston. The disadvantages of such shock absorbers include the complexity of the design, necessity to assure proper piston guidance and alignment and the necessity to seal both ends of the cylinder's casing in order to achieve proper operation.
Lately, the use of the elastomers has been permeated in the art of draft gears used in the ends of railway freight cars.
U.S. Pat. No. 6,446,820 to R. Barker et al teaches one type of draft gears utilizing a series of generally hollow elastomeric spring pads to absorb shocks in draft and buff conditions.
This type draft gear generally includes a rear compressible resilient member disposed between a rear follower and a yoke portion and a front compressible resilient member disposed within the yoke and associated with a coupler follower. A center rod extends through the yoke portion, through the rear compressible resilient member and through the rear follower. Both the front and rear compressible resilient members are comprised of a stack of elastomeric pads. The design does not utilize a steel housing and steel friction members, thus resulting in reduced draft gear assembly weight.
In draft, only the front stack of pads is loaded, as the yoke is moving toward the coupler follower which is abutting a pair of front stops, however in buff, both stacks of pads are loaded as the coupler follower is first moving toward the yoke and further causing the yoke to move in a direction of the rear follower which is abutting a pair of rear stops.
It has been discovered, such type draft gears have a low efficiency in retaining the absorbed shocks and therefore produces undesirable recoil action.
Therefore, it is desirable to employ simple shock absorbing devices providing improved reliability and reduced recoil action.